This invention relates to elastomer-based adhesive compositions.
The use of adhesives and sealants has been referred to throughout virtually all of recorded history. Sealants and adhesives have taken on even greater significance as society has become more industrialized. Natural rubber has long been a component of such compositions and still is. However, the advent of the synthetic polymer industry provided a variety of new sources of elastomeric components for such compositions. This has resulted in a high level of sophistication in the synthetic polymer industry thus allowing the tailoring of new polymers with specific characteristics. The ability to tailor polymers is especially great in that area known as xe2x80x9clivingxe2x80x9d polymers, where different monomers and coupling agents can be sequentially added to give many different configurations. For instance, an alkenyl arene such as styrene can be introduced into a polymerization zone to form a resinous aromatic block A (Step I polymerization), thereafter a conjugated diene such as isoprene can be added to give an elastomeric block B followed by coupling with a difunctional coupling agent, X, to give an ABXBA structure (two AB arms) which is generally depicted as A-B-A. Thus, the polymerized styrene units become the endblocks and the polymerized diene units the midblock.
A similar high level of sophistication has developed in the adhesive industry as a result of intensive research that has gone on in the past several decades. For instance, it is known to use an additive which is compatible with the elastomeric midblock portion of a block copolymer and a separate additive which is compatible with the resinous endblock portion. These can be used alone or in combination with an extender oil. It has sometimes been observed that block copolymers of styrene and either butadiene or isoprene perform better in adhesive formulations at higher styrene levels in the polymer. Thus, it has been suggested in the art to use copolymers wherein 25 to 50 wt % of the copolymer is derived from styrene and the remainder from the diene. Others have suggested 16 to 30 wt % styrene content. However, adhesive and elastic performance can vary dramatically depending on the type of block copolymer that is used.
It is an object of this invention to provide an adhesive composition which exhibits improved, i.e., lower set or lower stress relaxation; and
It is a further object of this invention to provide an improved method of formulating an adhesive composition.
In accordance with this invention, an adhesive composition is provided comprising an elastomeric block copolymer having relatively large aromatic resinous endblocks, a midblock compatible resin and oil, and optionally, an endblock compatible resin.